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Scales

Scales#

PyLabRobot supports the following scales:

  • Mettler Toledo WXS205SDU

Scales are controlled by the Scale class. This class takes a backend as an argument. The backend is responsible for communicating with the scale and is specific to the hardware being used.

from pylabrobot.scales import Scale
from pylabrobot.scales.mettler_toledo import MettlerToledoWXS205SDU

backend = MettlerToledoWXS205SDU(port="/dev/cu.usbserial-110") # take any ScaleBackend you want
scale = Scale(backend=backend, size_x=0, size_y=0, size_z=0)
await scale.setup()

The setup() method is used to initialize the scale. This is where the backend will connect to the scale and perform any necessary initialization.

The Scale class has a number of methods for controlling the scale and reading measurements. These are:

=(MettlerToledoWXS205SDU)

Mettler Toledo WXS205SDU#

The Mettler Toledo XS205 scale is controlled by the MettlerToledoWXS205SDU class. This scale is used by the Hamilton Liquid Verification Kit (LVK).

The scale comes with an RS-232 serial port. You’ll probably want to use a USB to serial adapter to connect it to your computer. Any $10 generic USB to serial adapter should work (e.g. something that uses FTDI).

Note that this scale has a ‘warm-up’ time after plugging in that is documented as 60-90 minutes by Mettler Toledo depending on the document you like at. In our experience, 30 minutes is sufficient. If you try to take a measurement before this time, you will likely get a “Command understood but currently not executable (balance is currently executing another command)” error. Sometimes plugging the power cord in and out will make things work faster.

backend = MettlerToledoWXS205SDU(port="/dev/cu.usbserial-110")
scale = Scale(backend=backend)
await scale.setup()
await scale.get_weight(timeout="stable")

0.00148

This scale provides various timeouts:

  • "stable": The time to wait for the scale to stabilize before reading the weight. Note that this may take a very long time to finish if the scale cannot take a stable reading. If you’re not using the air enclosure, even being near the scale can cause it to never stabilize.

  • 0: Read the value immediately

  • n>0: Try to get a stable value for n seconds. If the value is stable before n seconds, return it immediately. Otherwise, return the value after n seconds.

These parameters are available for get_weight(), tare(), and zero().

await scale.get_weight(timeout=0)

0.00148

Example: getting timing information#

Let’s say you wanted to determine how long it takes to take a measurement. In PyLabRobot, this is easy:

backend = MettlerToledoWXS205SDU(port="/dev/cu.usbserial-110")
s = Scale(backend=backend)
await s.setup()

import time
import numpy as np

l = []
for i in range(10):
  t0 = time.monotonic_ns()
  await scale.get_weight(timeout="stable")
  t1 = time.monotonic_ns()
  l.append((t1 - t0) / 1e6)

print(f"{np.mean(l):.2f} ms ± {np.std(l):.2f} ms")

100.44 ms ± 6.78 ms
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